I know I am late to the party here. I’ve had this problem recently. I’m using the 2.x version.
The solution I discovered was to go to select the possible offending post and then to select the HTML source editor from the toolbar above the content window. A window should pop up and in the corner there should be a checkbox for “Word Wrap”. Make sure this is checked. This fits the HTML code to the width set in the theme and allows the sidebar to float where it needs to be.
